Rates of Karyotype Evolution

Abstract

Haiduk and Baker (1982) gave an insightful interpretation of rate variation in karyotype evolution. Their descriptive nomenclature, however, is misleading-karyotypic (Baker and Bickham, 1980) is an inappropriate term to apply to what Haiduk and Baker recognize as simply an exceedingly rapid rate of karyotype evolution. Simpson (1944) coined megaevolution to defiote the emergence of taxonomic above the genus level, contrasting it with both microevolution (below the species level) and (above the species level). By 1953, however, Simpson had abandoned the term, as it served merely to obscure the continuity of process along the evolutionary hierarchy. To resurrect an obsolete term originally used to refer to the development of really high [taxonomic] categories (Simpson, 1953:339) and reapply it to radical reorganization of the banding patterns ... while other closely related species retained the primitive condition (Haiduk and Baker, 1982) is, I think, inappropriate and somewhat misleading. The phenomena to be denoted are fairly well-known, and perhaps most graphically exemplified within the genus Muntiacus (Cervidae). In M. reevesi both sexes have 2n = 46, while M. muntjak has 2n = 6 in the female and 2n = 7 in the male. This great cytological disparity occurs in spite of overwhelming morphological similarity and hybridization potential between the species (Wurster and Benirschke, 1970; Fredga, 1977; Shi Liming et al., 1980). Similar situations are also known for some groups of rodents (White, 1978; Elder, 1980), foxes (Bush, 1975), insectivores (Arnason, 1972), horses (Ryder et al., 1978), and gibbons (Marks, 1982; Shafer et al., 1983; van Tuinen and Ledbetter, 1983). The inference to be drawn is that the evolution of the chromosomes in these groups has proceeded much more rapidly than the evolution of either the morphology of these organisms or their genes. What needs to be recognized is that the karyotype represents a level of genomic organization and evolution phenomenologically distinct from the genotype and from the phenotype with which it is associated. Phrased another way, chromosomes are seen to have evolved according to rules of their own not clearly related to any properties of the organisms whose heredity they ... [are] supposed to be carrying (Darlington, 1978:447). If one wishes to call attention to the variation across taxa in the evolutionary rates of karyotypes, Simpson (1944) coined a term that can be transferred and applied to the level: tachytely, by which he denoted exceedingly rapid evolution within a group. I suggest, therefore, that if it is necessary to give a name to the phenomenon of high diversity among closely related species, chromosomal is a phrase more meaningful, accurate, and consistent with evolutionary terminology than karyotypic megaevolution. The application of Simpson's to the level of evolution is obviously an analogy, and it must differ from his original formulation in two important respects: (1) Simpson plotted a measure of macroevolution (the survivorship of supraspecific taxa) versus absolute time; and (2) he defined tachytely relative to a modal rate of evolution for a group, horotely. Unfortunately, for the study of rates of evolution, we have no measure of absolute time against which to plot chromosome rearrangement; the fossil record reveals nothing about the karyotype of the fossil animal. Further, there is presently no useful way to measure accurately the amount of karyo-